1. Field of the Invention
The present invention relates to a particle measuring apparatus, and more particularly to a particle measuring apparatus for surrounding a sample with a sheath liquid to hydrodynamically converge and flow them by giving them a predetermined pressure difference, and making a measurement based on a signal data from a particle in the sample.
2. Description of the Background Art
A particle measuring apparatus of this type, such as a flow cytometer, is an apparatus for allowing the sample and the surrounding sheath liquid such as a physiological saline solution to flow at a high speed in a flow cell by giving them a predetermined pressure difference, converging the flow hydrodynamically to form a sample flow, applying a laser light to specimen particles flowing one by one at a converging position, detecting a scattered light from a solid component of the specimen particle by a photodetector, and analyzing a property, structure, and the like of the specimen particle from an obtained pulse waveform. Such an apparatus is used in the fields of cytology, hematology, oncology, genetics, and the like. Naturally, an accurate measurement of particles in the sample would be difficult if impurities (for example, a bacterium or an air bubble in the case where the objects of measurement are bacteria in a urine) are mingled in the sheath liquid.
In order to solve this problem, a method is known in which the impurities are removed by providing a filter section between a flow cell and a sheath liquid pressuring section which supplies a pressurized sheath liquid to the flow cell. However, when the higher pressure side of the filter section is clogged with impurities, the filtration efficiency decreases and the filter section must be exchanged. Therefore, a method is known in which a pressure sensor is provided downstream of the filter section to check the decrease in pressure of the sheath liquid, thereby monitoring the clogging of the filter section or, further, adjusting and controlling the pressurizing force of the sheath liquid pressurizing section on the basis of a pressure detection signal of the pressure sensor (for example, see Japanese Examined Patent Publication No. HEI 05(1993)-87779).
However, even if the clogging of the filter section can be monitored in this way by providing a pressure sensor downstream of the filter section, there is a problem that the pressure change caused by clogging is very small, so that, when the pressure change has been detected, the filter section is almost completely clogged, rendering the filter section impractical. Also, it is known that, even if there is no clogging in the filter section and the pressurizing force is constant, the flow rate (amount of flow per unit period of time) changes greatly because of the change in kinematic viscosity of the liquid due to the change in temperature. Therefore, at present, it is difficult to maintain a constant flow rate of the sheath liquid by monitoring or controlling the filter section in accordance with the pressure change. Also, very high costs of the pressure sensors and the pressure controlling means (for example, automatically controlled air pressurizing device) are problems.